Our ancestors arrived much earlier than thought

A fossil lower jaw found in Ethiopia has pushed back evidence for the earliest human genus -- Homo -- to 2.8 million years ago, around 400,000 years earlier than previously thought.

The new fossil unearthed at Ledi-Geraru research area in Ethiopia provides clues to changes in the jaw and teeth in Homo only 200,000 years after the last known occurrence of Australopithecus afarensis ("Lucy").

Found by team member and Arizona State University (ASU) graduate student Chalachew Seyoum, the Ledi-Geraru fossil preserves the left side of the lower jaw, or mandible, along with five teeth.

The fossil analysis revealed advanced features, for example, slim molars, symmetrical premolars and an evenly proportioned jaw, that distinguish early species on the Homo lineage.

"In spite of a lot of searching, fossils on the Homo lineage older than 2 million years ago are very rare," said Brian A. Villmoare of the University of Nevada, Las Vegas.

"To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting," Villmoare said.

"The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo," said William H. Kimbel, director of ASU's Institute of Human Origins.

"It's an excellent case of a transitional fossil in a critical time period in human evolution," Kimbel said.

Climate change that led to increased African aridity about 2.8 million years ago is often thought to have stimulated species appearances and extinctions, including the origin of Homo.